Fractal analysis of eye movements during reading.

We present a new method for the analysis of reading eye movements based on the methods of nonlinear dynamics. In this preliminary study, the eye movements of normal and abnormal readers were analyzed for evidence of chaotic, nonlinear dynamical behavior. Both power spectral density analysis and fractal dimension determination showed evidence of nonlinearity as manifest in chaotic behavior. The computed fractal dimension of the system's presumed attractor seemed directly related to qualitative assessment of reading ability. Representative subjects did not differ in a similar analysis of pursuit movements. Although fractal analysis did not distinguish unconditionally between normal and abnormal reading in this preliminary study, it nevertheless offers a promising hitherto unused approach to the analysis of eye movements and the modeling of oculomotor behavior.

Detection of ophthalmoscopically occult maculopathy by focal electroretinography.

We performed focal electroretinography (FERG) in 10 patients with reduced vision of unknown cause. No patient gave a family history of uncorrectably reduced vision, and none had taken any drug known to damage the retina. Visual acuity claims ranged from 20/40 to finger counting. Color discrimination was reduced in all but one patient. Visual field sensitivity was reduced nonspecifically in five of the six patients who could perform perimetry reliably. Ophthalmoscopy showed no pertinent abnormality in four of 10 patients, mildly reduced foveal reflex in two of 10, arteriolar narrowing in two of 10, foveal pigment disturbance in one of 10, and preretinal gliosis in one of 10. Flash visual evoked potential (VEP) testing showed no abnormality in seven of 7 patients. The pattern VEP was reduced or delayed in five of 5 patients, only one of whom showed the expected amplitude loss with decreasing check size. Five patients completed full-field electroretinography, six patients completed fluorescein angiography, and none showed pertinent abnormality on either test. FERG was performed in both eyes of seven patients and in one eye of three others. Signals were non-recordable from four eyes, abnormal in nine, equivocal in three, and normal in one. The severity of FERG abnormality did not correlate with the severity of acuity loss. FERG abnormalities were found in fourteen of 17 eyes of 10 patients with vision loss out of proportion to clinical findings. In these patients only one of 5 pattern VEP tests, none of 5 full-field ERGs, and none of 7 fluorescein angiograms suggested correspondingly severe maculopathy. The FERG results in these cases mitigated suspicions of nonorganic vision loss and permitted speedier referral to retinal disease and low vision specialists.

Corneal topographic changes after extraocular muscle surgery.

PURPOSE: Changes in refractive error have been reported after strabismus surgery. The influence of extraocular muscle tension on corneal topography is thought to be an important mechanism contributing to this alteration. This study investigates topographic changes after strabismus surgery in human beings. METHODS: The corneal topography of 63 eyes of 43 patients who underwent either strabismus or optic nerve sheath fenestration (ONSF) surgery was measured before and after operation. The ONSF surgery involved removing and reattaching the medial rectus muscle, thus acting as a control for the strabismus procedures. Corneal power was measured centrally and at 1.5 and 3.0 mm in 8 meridians. Analysis of variance was performed to compare the difference in preoperative and post-operative corneal power for all procedures and comparing recessions, resections, combined recession and resection, and ONSF as grouped procedures. RESULTS: All groups showed a significant change in preoperative to postoperative corneal power (P< .01). A significant change was seen between all grouped procedures except when the resection was compared with ONSF and compared with recession. CONCLUSIONS: Change in the tension of 1 muscle does not produce a change in the adjacent quadrant as much as it produces a significant change in the entire corneal surface, which illustrates the interaction and interdependence all corneal positions have with respect to each other.

Visual system effects of exercise on Mauna Kea at 2,200 and 4,200 meters altitude.

Field exercise studies were performed at two altitudes (2,200 and 4,200 m) in 2 successive years using different sets of young male volunteers. Visual function indices were measured both at sea level and during a strenuous exercise regime at altitude. Volunteers were grouped in the first study by initial rest period (2 days vs. no rest) and in the second by diet (supplemental carbohydrates vs. Meals Ready to Eat rations only). Overall results showed no effect according to grouping, a decrease in average visual acuity at the higher altitude overall, and a decrease in electroretinographic (ERG) photopic flicker responses at moderate altitude. It is concluded that heavy exercise at these altitudes may not have operationally significant effects on ground troops in night vision or target recognition, although the change in ERG parameters does indicate a shift in retinal cone physiology that may have subtler effects.

Evaluation of a delivery system providing long-term release of cyclosporine.

OBJECTIVES: To examine the clearance of cyclosporine after intravitreal injection and to assess the kinetics and toxic effects of an intravitreal device that provides sustained delivery of cyclosporine. METHODS: Rabbits were divided into two groups to evaluate (1) the elimination kinetics after 1-microgram and 10-microgram intravitreal injections of cyclosporine and (2) the levels produced after implantation of a device that contained cyclosporine over 6 months. The toxic effects of the intravitreal device over 6 months were assessed in rabbits and cynomolgus monkeys. RESULTS: After the 10-microgram injection, the half-life was longer (10.8 hours vs. 4.2 hours) and the distribution volume was smaller (1.7 mL vs 3.2 mL) than after the 1-microgram injection. This difference can be attributed to saturable partitioning of the drug. The device resulted in a vitreous concentration of approximately 500 ng/mL throughout the study period. In the rabbit it resulted in reversible lens opacification and decreased b-wave amplitude. This toxic effect was not detected in the monkey. CONCLUSIONS: The device produces sustained intravitreal levels of cyclosporine. Although it was associated with reversible toxic effects in the rabbit, it was well tolerated in primates. Sustained-release implants are a promising new treatment for chronic uveitis.

Visual system electrodiagnosis in neurologic disease of childhood.

Electrodiagnostic examination of the visual system includes visual evoked potentials and electroretinography. These tests can be applied to all ages. Pitfalls in the clinical ophthalmologic examination of children, such as optic nerve pallor and pigmentary retinopathy, can be clarified by these tests. Investigation of neurodegenerative disease may be directed to the most likely defect by results of visual evoked potential testing and electroretinography. The child who is unable to meaningfully communicate sensory experience can be objectively evaluated by these studies. Familiarity with the applications and limitations of these tests will allow the examiner to choose the appropriate setting and time to apply each of them.

Dichoptic luminance beat visual evoked potentials in the assessment of binocularity in children.

Direct evidence of a distinct cortical binocular pathway has been provided by the production of nonlinear (difference) beats from dichoptic luminance stimulation in stereonormal adults and the absence or diminution of these beats in stereoblind subjects. We have investigated a clinically useful application of this technique in a pediatric population with potentially abnormal binocular vision. We recorded dichoptic luminance beat visual evoked potentials (VEPs) from 20 children (ages 7 months to 8 years) with abnormal binocular ability secondary to strabismus and/or amblyopia and compared this to a control group of 20 children with normal binocularity. Stereoblind children generated significantly lower dichoptic signal-to-noise ratios than stereonormal children (P < .001). Responses to monoptic multifrequency flicker were not significantly different between the two groups (P = .936). This dichoptic VEP can be performed quickly and easily on young children and gives a quantitative assessment of cortical binocularity that may not be determinable by standard clinical methods. This technique may also prove useful for the preoperative gradation of binocular potential and prediction of postoperative binocular fusion.

Fractal analysis of steady-state-flicker visual evoked potentials: feasibility.

Electrophysiological models of visual evoked potential recording have assumed that response variability is caused predominantly by random noise added to a true steady signal. Since neuronal geometry has a fractal structure, neural activity may demonstrate deterministic nonlinear dynamics, i.e., chaos. We recorded several-minute time-series traces of the visual evoked potential magnitude in response to full-field flicker from three glaucoma patients and one normal subject. When plotted in phase space, the steady-state response derived from a lock-in amplifier shows an apparent so-called strange attractor (extended nonrepeating loops) rather than the pattern expected from a signal-plus-noise model (a fuzzy dot). The fractal dimension of this attractor may be a more sensitive indicator of early optic-nerve damage than are visual evoked potential latency or amplitude measures.

Acute laser lesion effects on acuity sweep VEPs.

Q-switched neodymium-YAG (infrared) laser lesions at energies up to and including retinal hemorrhages were placed under visual control in the parafovea and the fovea of anesthetized monkeys. Visual-evoked potential (VEP) data were obtained by parallel analog (vector voltmeter) techniques from scalp electrodes in response to high luminance counterphasing sine wave gratings. The gratings were swept downward in spatial frequency to determine an acuity estimate by recording of the VEP magnitude increase onset. Acuity estimates were determined immediately post-exposure and at 15 sec intervals up to 12 min. These were analyzed as a function of laser exposure site and retinal lesion produced. Significant delays in VEP lock-in were demonstrated in subjects that had parafoveal burns or parafoveal subretinal hemorrhages. Foveal burns caused severe short-term fluctuations before a sustained decrease in acuity. Contained foveal hemorrhages produced sustained acuity losses. Foveal exposures that did not produce an immediately visible lesion did not produce measurable changes in VEP response lock-in time. These results probably are independent of visible flash effects and indicate that there may be a transient neural shock effect from parafoveal lesions that can affect the fovea.

Flicker visual evoked potential differentiation of glaucoma.

We have been recording visual evoked potentials (VEP's) produced by a series of increasing rate flicker stimuli to find early neuronal loss in chronic open-angle glaucoma patients and suspects. This technique has demonstrated selective early signal losses in high flicker frequency responses (25 and 31 Hz). This signal loss can precede the characteristic arcuate losses found with automated perimetry. In this retrospective study, 428 visual fields were graded and grouped with their associated cup/disk ratios, peak recorded intraocular pressures (IOP's), and flicker VEP measures in a population of patients either diagnosed with glaucoma or at risk for glaucoma (glaucoma suspects). The correlation of selective high frequency (31 Hz) flicker VEP amplitude loss with visual field damage was confirmed. More severe VEP losses correlate well with optic cup size and visual field grade, but not with peak recorded IOP. Phase angle data did not differentiate between populations separated by visual field losses and VEP amplitude measurements.

An evaluation of four multifocal contact lenses in young monocular aphakic patients.

To evaluate the efficacy of multifocal contact lenses in the correction of presbyopic symptoms we fit six young monocular aphakic patients with four different bifocal contact lenses (ACC, Tangent Streak, VFL, and Constavu). Each patient had 20/20 uncorrected vision and normal accommodation in the other eye. We evaluated both subjective and objective parameters of comfort and vision for all four lenses. We found no significant overall performance advantage among the four lenses based on comfort, visual satisfaction, visual acuity, contrast sensitivity, or wear-related corneal complications.

Intravitreal sustained-release ganciclovir.

Current treatment of cytomegalovirus retinitis in patients with acquired immunodeficiency syndrome involves frequent intravenous administration of sodium ganciclovir that often results in unacceptable side effects. We have developed devices that release ganciclovir at rates of 2 micrograms/h and 5 micrograms/h in vitro. When implanted into the vitreous of rabbit eyes, mean intravitreal ganciclovir levels of 9 mg/L and 16 mg/L were maintained for more than 80 and 42 days, respectively. Devices were well tolerated, with no toxic effects attributable to the polymers used in the devices. This investigation indicates that these devices can maintain therapeutic levels of drug for extended periods and are well tolerated in the rabbit eye. They may prove useful in the clinical management of cytomegalovirus retinitis in patients with acquired immunodeficiency syndrome.

The electroretinogram in neurologic practice.

The ERG can be helpful to the neurologist who must evaluate patients with known or suspected retinal disease. The test is most useful in detecting (or excluding) outer retinal disease, especially in presymptomatic, very young, or mentally enfeebled patients. It can also help in interpreting the meaning of an abnormal VEP.

High-frequency flicker visual-evoked potential losses in glaucoma.

Anatomic evidence indicates that glaucoma initially destroys large axons before affecting smaller ones. Large axonal size has been correlated with fast, transiently responding retinal ganglion cells (alpha/Y cells). A second population of cells (X cells) does not follow flicker stimulation to as high a frequency as Y cells. The luminance flicker visual-evoked potential (VEP) shows two response maxima as a function of temporal frequency. These two response peaks may indicate driving of the flicker VEP by two separate neuronal populations, possibly X and Y cells. The authors have recorded flicker VEPs as a function of frequency from both normal subjects and glaucoma patients with asymmetric visual field loss in various stages of their disease. The data obtained demonstrate loss of high flicker rate responses apparently preceding and correlated with perimetric field loss and stage of glaucoma. Flicker VEP responses below 13 Hz tend to be preserved, whereas above this frequency the response attenuation is directly proportional to the severity of visual loss. This technique may provide an early warning of ganglion cell loss in the early stages of this disease.

Laser-induced chromatic adaptation.

Detecting a target in a visually noisy back-ground depends on the ability of the observer to discriminate the target from the surrounding terrain. Visible laser irradiation at less than damage levels may act as a masking source by reducing the observer's ability to resolve differences in the visual scene. The experiment reported here specifically investigates the comparability of shuttered CW and Q-switched visible lasers to alter/degrade color discrimination. Visual evoked potentials (VEP's) were used to examine the short time course effects in monkeys of luminance-matched flashes from a 694 nm ruby Q-switched laser and 100 ms shuttered krypton CW laser (676, 568, and 531 nm lines). The test stimulus was a shifting pattern of alternating luminance-matched 510 and 550 nm green bars. With flashes equated to 4.8 log T-s, similar flash effect curves were seen, demonstrating 1.5-s changes in response magnitude. This level of flash did not extinguish the response to the stimulus. The flash effects curve was "W"-shaped, with an intermediate signal peak occurring at approximately 500 ms after the flash and whose level exceeded the baseline magnitude. The hypothesized mechanism for this result is an induced luminance imbalance caused by a transient shift in the peak color responsiveness of the visual system, which recovers with two different time constants. It is concluded that red and green colored laser flashes shift the color balance transiently in the visual system (yellow flashes to a lesser extent); thus, targets may change both hue and brightness after an observer receives colored flashes.(ABSTRACT TRUNCATED AT 250 WORDS)

Laser flash effects on laser speckle shift visual evoked potential.

Steady-state visual evoked potentials (VEP's) were recorded from four cynomolgus monkeys in response to a sinusoidally oscillating 10 degrees helium-neon laser speckle field (632.8 nm), moving vertically 2.5 degrees at 8 shifts per second. A 5-pulse flash train at the maximum permissible exposure (MPE) dose from a collimated Q-switched frequency-doubled neodymium laser (532 nm) was superimposed on the foveal stimulus and the subsequent disruption and recovery of the VEP measured. Minimal disruption of the response signal magnitude was demonstrated (0.1 greater than p greater than 0.05) which recovered within 300 ms of the termination of the pulse train. A small but significant (p less than 0.01) disruption of phase entrainment was also noted that recovered within the same period. This is contrasted with a second experiment with three monkeys in which an argon (514 nm) laser served both as the speckle stimulus source and as the shuttered flash. Exposure to collimated MPE argon radiation for 250 ms immediately depressed the VEP (97%, p less than 0.01) and showed recovery to 70% of the pre-flash baseline only after 3 s. Phase lock was also severely degraded for several seconds. These results imply that visual processing of nonacuity-limited medium contrast stimuli with broad spatial frequency content will probably not be materially affected by ultra-short pulsed laser exposure at these energy levels and frequencies. However, even safe levels of collimated continuous laser light may have severe effects on vision that could parallel flash effects seen with Xenon discharge flash lamps.

Repetitively pulsed small spot light flashes. Depression of steady-state pattern visual evoked potentials.

Steady-state visual evoked potentials (VEP's) were elicited from human volunteers by contrast reversal of a 43% contrast checkerboard pattern resolvable with 6/24 (20/80) Snellen acuity. Superimposed on the fixation point was a Xenon flash lamp subtending 0.358 degrees visual angle, which produced a 0.05 J/cm2 sr flash in 2 microseconds (full width, half maximum). Single pulses of light had no significant effect on the VEP. Increasingly significant decreases in VEP signal magnitude were noted with 2 and 5 pulses of 20 Hz white light. No disruption of VEP phase entrainment was noted with any of the flash exposures. Partial reciprocity was demonstrated between number of flashes at this frequency and response decrement.

Flicker electroretinograms and visual evoked potentials in the evaluation of laser flash effects.

Electroretinograms (ERG's) and visual evoked potentials (VEP's) were recorded from four cynomolgus monkeys in response to a sinusoidally flickering argon laser beam (514 nm) producing a 50-micron spot on the fovea. Super-position of a 20-Hz train of six pulses of Q-switched (120 ns) frequency-doubled neodymium laser light (532 nm) at "safe" exposure energies in 250 ms had no significant effects on the ERG (p greater than 0.05). The VEP was disrupted significantly (p less than 0.001) but demonstrated recovery within 500 ms of the initial pulse. Therefore, flash effects of pulsed visible lasers at these doses on suprathreshold luminance processing are probably limited only to the exposure period.

Foveal xenon flash disruption of steady-state visual evoked potentials.

The effects of foveal exposure to near maximum permissible levels of broad-band light from lamps of differing flash durations and visual angle subtense on steady-state visual evoked potentials ( SSVEP 's) were measured in four human volunteers. SSVEP 's in response to a counterphasing checkerboard pattern were recorded with a two-phase vector voltmeter used as an analog fourier analyzer and were averaged across repeated presentation of the flash from each lamp. Two microsecond flashes of approximately 0.4 degrees have minimal, although significant effects on early visual processing of a 2.77 degrees diameter 6/24 (20/80) target. Five hundred microsecond flashes of equivalent size have greater effects; larger flashes (11.3, degrees 500 mus) produced afterimages that totally obscured the target for approximately 3.4 s and also reduced the SSVEP to near zero. Recovery of the SSVEP was subjectively correlated with afterimage duration rather than target obscuration . Therefore, single short small retinal image flashes may have minimal transient effects on photopic suprathreshold vision.